ZURICH, July 25 (Reuters) - Almost 30,000 Britons and Austrians have come forward to pay tax on previously undisclosed Swiss bank accounts under bilateral agreements aimed at rooting out untaxed money in Switzerland.

Switzerland's Federal Tax Administration (FTA) said on Thursday it had transferred a first tranche of 258.3 million pounds ($396.7 million) to Britain based on 14,789 declarations, and 416.7 million euros ($551.6 million) to Austria based on 13,592 clients.

Swiss secrecy laws have helped to make the country the world's biggest offshore financial centre, but have also drawn the ire of countries seeking to fight tax evasion.

Under the deals with Britain and Austria, clients with undisclosed Swiss bank accounts have the option either to provide information to the relevant tax authorities and pay any monies due, or make a punitive one-off payment in exchange for retaining their anonymity.

Thereafter taxes will be levied on the capital gains and income generated by the accounts, to be passed on to the relevant governments.

However, after Germany rejected a similar arrangement in December, Swiss banks have become less enthusiastic about a model they see as cumbersome, and which puts the onus of tax collection onto them.

With Swiss attitudes on automatic exchanges of information softening, more clients than expected are also choosing to pass account details direct to their tax authorities, rather than making the punitive payment to remain anonymous.

"Only a comparatively small group of individuals opted for the regularisation of assets by means of the one-off payment," said the Swiss Bankers Association, confirming its analysis from earlier in July. (Full Story)

Under a deal signed on Jan. 1, Swiss banks paid 500 million francs to Britain, which they will only receive back in full if their UK-resident clients pay at least 1.3 billion francs through the anonymity scheme, rather than direct to Britain.

Britain's Office for Budget Responsibility has said it could rake in 3.2 billion pounds this year from individual payments and the anonymity scheme combined, but has raised doubts about the timing of payments after the latter scheme took in less than expected in the first part of the year.

According to an estimate by Imke Gerdes from global law firm Baker & McKenzie, Austria is expected to raise 1 billion euros in tax revenue in 2013 from its agreement with Switzerland.

Switzerland's State Secretariat for International Financial Matters said implementation of the agreements had not thrown up any major obstacles, adding that negotiations on similar deals are currently underway with Greece and Italy.